A system and method for controlling exhaust gas recirculation (EGR) of an internal combustion engine is described in U.S. Pat. No. 7,043,914. The patent describes an EGR system that has a high pressure loop and a low pressure loop. The system uses two separate EGR valves and two separate EGR coolers to provide exhaust gases from upstream a turbocharger turbine to a location downstream the turbocharger compressor or to a location upstream the turbocharger compressor. The system also includes a particulate filter to protect the turbocharger compressor from engine soot. The system purportedly allows large amounts of EGR when the engine is operated at high engine speed and high engine load, thereby reducing engine NOx.
The above-mentioned system can also have several disadvantages. Namely, the system requires control of two different EGR valves. Controlling two different EGR valve may be difficult because the two valves may have different flow characteristics due to deposits and manufacturing tolerances. As a result, EGR rate and engine torque may vary or be unstable when both valves are simultaneously controlled as the engine control system may not have sufficient information to determine which valve to adjust. In addition, switching EGR delivered upstream the compressor to downstream the compressor can cause EGR flow variation in one EGR valve when the other EGR valve is opened or closed. Further, the system requires two EGR coolers. Further still, the system enables EGR flow through the compressor simply in response to a temperature sensor and will only allow EGR flow in that path if the temperatures are high enough to regenerate the particulate filter. As a result, the engine may operate for a period of time when little EGR flows to the engine because the desired EGR flow rate can only be generated in the pre-turbine to pre-compressor path; however, low exhaust temperatures will cause flow to be inhibited.
The inventors herein have recognized the above-mentioned disadvantages and have developed a system and method that offers substantial improvements.